Bicarbonate, sulfate, and chloride water in a shallow, clastic-dominated coastal flow system, Argentina

LOGAN, W. S.; AUGE, M. P.; PANARELLO, H.0.

GROUND WATER

Blackwell Publishing

Lugar: Westerville. EE.UU.; Año: 1999 vol. 37 p. 287 - 295

0017-467X

Abstract Most of the cities southeast of Buenos Aires, Argentina, depend heavily on ground water for water supply. Whereas ground water quality is generally good in the region, economic development along the coastal plain has been constrained by high salinities. Fifty-four wells were sampled for major ions in zones of recharge, transport and discharge in an area near La Plata, 50 km south­east of Buenos Aires. The shallow, southwest to northeast coastal flow system is >30 km long but is only 50 to 80 m thick. It con­sists of Plio-Pleistocene fluvial sand overlain by Pleistocene eolian and fluvial silt and Holocene estuarine silty clay. Hydrochemical endmembers include HC03, SO4, and CI water. Bicarbonate-type water includes high plain recharge water (Ca­Na-HC03) that evolves through cation exchange and calcite dissolution to a high pH, pure Na-HC03 endmember at the southwest edge of the coastal plain. Similar Na-HC03 water is also found underlying recharge areas of the central coastal plain, and a lens of Ca-HC03 water is associated with a ridge of shell debris parallel to the coast. Mixed cation-Cl water near the coastline represents intruded sea water that has undergone cation exchange. Chemically sim­ilar water underlying the southwest coastal plain, however, can be shown isotopically to have formed from fairly dilute solutions concentrated many times by evapotranspiration. In the central coastal plain, Na-SO4-Cl water (up to 17,000 mg/L SO4) underlies hummocks and, at depth, forms plumes in the subsurface. The chemical controls are dissolution of gypsum precipitated during pyrite oxidation, evapotranspiration, and calcite precipitation. This extreme diversity of water chemistry in a shallow, clastic flow system is uncommon. It is made possible by (1) the pres­ence of oxidized, originally pyritic, but slightly permeable sediments that have been exposed to evolving geochemical and hydro­dynamic conditions, and (2) highly focused recharge that favors the development of fresh water bodies underlying recharge zones and stagnant, evaporated water underlying other areas.